Metal powder, green compact and production method thereof

ABSTRACT

A metal powder becoming a raw material for obtaining a green compact by compacting, the metal powder having a plurality of accessible surfaces allowing for surface contact of adjacent metal powders with each other when filled, and a method for producing a green compact by compacting the metal powder, the method comprising the steps of: a charging step of charging the metal powder into a predetermined die, a compacting step of filling the metal powder in the die, and a compacting step of compacting the metal powder to obtain the green compact.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a metal powder as a raw material for obtaining a green compact by compacting, a green compact using the same and a production method thereof.

2. Description of the Related Art

A green compact obtained by compacting a metal powder (hereinafter, sometimes simply referred to as a “powder”) is used, for example, as a material for a sprocket of gear parts or for a rotor or vane of pump parts, and attempts are being made to increase the density of such a green compact to as high as possible for enhancing a performance such as mechanical strength.

The method for increasing the density of a green compact has been conventionally disclosed, for example, in Japanese Unexamined Patent Publication (kokai) Nos. 6-2007, 10-140207, 10-219302, 11-100602, 2001-254102, 2002-317204, 2003-171741 and 2004-342937.

For example, the approach from a material aspect includes a method of irregularly shaping the powder to enhance the compaction property of the powder. However, the enhancement in compaction property of a powder is limited and the density of a green compact may not be sufficiently increased.

Also, a method of enhancing the filling property of powder by using a mixture of powders differing in the particle size is known. However, in this case, the gap formed between powders filled may not be sufficiently reduced. A method using a spherical powder is also known, but in this case, the compaction property of powder may decrease. Therefore, in any of these methods, the density of a green compact may not be satisfactorily increased.

The approach from a compacting aspect includes a compacting method of applying a high pressure. However, in this case, the quality of a green compact may deteriorate after compacting. A compacting method using a lubricant is also known, but this has a problem that the lubricant remains inside the green compact after compacting and the density of a green compact may not be satisfactorily increased.

That is, in conventional methods, the density of a green compact cannot be satisfactorily increased or a further increase in the density can be hardly realized.

The green compact includes a powder magnetic core obtained by compacting a metal powder of which surface is coated with an insulating film. Such a powder magnetic core is applied to a material placing importance on magnetic characteristics and is used, for example, as a material of power source parts such as noise filters and reactors. In addition to these uses, application of the powder magnetic core to a motor core, a solenoid core or the like has been recently proposed by making use of the degree of freedom of its shape.

The great characteristic feature of the powder magnetic core includes a magnetic flux density and an iron loss. When a powder magnetic core is applied to a motor core or the like, a higher output is obtained as the magnetic flux density is higher, and a higher-efficiency motor is obtained as the iron loss is lower. In order to increase the magnetic flux density, it is important to increase the density of the powder magnetic core. On the other hand, in order to decrease the iron loss, it is important to decrease the eddy loss in an AC magnetic field and furthermore decrease the hysteresis loss generated due to deformation (distortion) of the powder at compacting.

Conventionally, a technique of coating the powder surface with an insulating film and thereby ensuring electrical insulation (hereinafter simply referred to as “insulation”) between powders is employed for the purpose of decreasing the eddy loss. On the other hand, in order to obtain a high magnetic flux density, compacting at a high pressure and reduction in the thickness of the insulating film are necessary. However, the high-pressure compacting may cause rupture of the insulating film due to deformation of the powder or friction, and the reduced thickness of the insulating film may bring about extreme decrease in the insulation between powders. As a result, the eddy loss or hysteresis loss is disadvantageously increased.

Accordingly, a powder for powder magnetic cores, which can satisfy both high magnetic flux density and low iron loss of the powder magnetic core, a powder magnetic core using the powder, and a production method thereof have been recently disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) Nos. 2000-169901, 2001-155914, 2003-303711, 2003-332116, 2004-14614, 2004-221549, 2005-113258 and 2005-213639. However, in these patent publications, either a magnetic flux density or an iron loss is not satisfied. That is, it is difficult in conventional methods to satisfy both high magnetic flux density and low iron loss.

SUMMARY OF THE INVENTION

The present invention has been made by taking into consideration these conventional problems and an object of the present invention is to provide a metal powder having high filling property and capable of realizing a high-density green compact, a green compact using the same, and a production method thereof.

A first invention is a metal powder which is a raw material for obtaining a green compact by compacting.

The metal powder is a metal powder characterized by having a plurality of accessible surfaces allowing for surface contact of adjacent metal powders with each other when filled.

As described above, the metal powder of the present invention has a plurality of accessible surfaces allowing for surface contact of adjacent metal powders with each other when filled. That is, when the metal powder is filled, the metal powder can unfailingly make surface contact with the adjacent metal powder at a plurality of accessible surfaces, whereby the gap formed between these metal powders when filled can be reduced and the filling property of the metal powder can be enhanced.

A green compact obtained by densely filling this metal powder having high filling property and compacting it in the state of the gap between metal powders being reduced comes to have a high density with the presence of fewer voids after compacting.

Also, as the metal powder has high filling property, a high pressure for filling the gap between metal powders need not be applied at compacting, so that a high-density green compact can be obtained by performing the compacting at a pressure lower than ever.

In this way, according to the present invention, a metal powder having high filling property and capable of realizing a high-density green compact can be provided.

A second invention is a method for producing a green compact by compacting the metal powder of the first invention, the method comprising:

a charging step of charging the metal powder into a predetermined die,

a filling step of filling the metal powder in the die, and

a compacting step of compacting the metal powder to obtain the green compact.

In the production method for a green compact of the present invention, the metal powder of the first invention, that is, the metal powder with high filling property is used and therefore, a green compact obtained according to this production method by densely filling the metal powder in a predetermined die and compacting it has a high density.

Also, as the metal powder has high filling property, a high pressure for filling the gap between metal powders need not be applied in the compacting step, so that a high-density green compact can be obtained by performing the compacting at a pressure lower than ever.

In this way, according to the production method of the present invention, a high-density green compact can be obtained.

A third invention is a green compact produced by the method for producing a green compact of the second invention.

The green compact of the present invention is produced by the method for producing a powder compact of the second invention and therefore, the green compact comes to have a high density.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view showing the metal powder in Example 1,

FIG. 2 is cross-sectional enlarged view showing the surface of the metal powder in Example 1,

FIG. 3 is a cross-sectional enlarged view showing the surface of the metal powder in Example 1,

FIG. 4 is an explanatory view showing the filled state of the metal powder in Example 1,

FIG. 5 is an explanatory view showing the green compact in Example 1,

FIG. 6 is an explanatory view showing the metal powder having other shape in Example 1,

FIG. 7 is an explanatory view showing the metal powder having other shape in Example 1,

FIG. 8 is an explanatory view showing the metal powder having other shape in Example 1,

FIG. 9 is an explanatory view showing the gear part (sprocket) in Example 2,

FIG. 10 is an explanatory view showing the pump part (rotor) in Example 2,

FIG. 11 is an explanatory view showing the powder for powder magnetic cores in Example 3,

FIG. 12 is a cross-sectional enlarged view showing the surface of the powder for powder magnetic cores in Example 3,

FIG. 13 is a cross-sectional enlarged view showing the surface of the powder for powder magnetic cores in Example 3,

FIG. 14 is an explanatory view showing the filled state of the powder for powder magnetic cores in Example 3, and

FIG. 15 is an explanatory view showing the powder magnetic core in Example 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the first invention, the total area of the accessible surfaces is preferably 70% or more of the entire surface area of the metal powder.

In this case, the metal powder when filled can ensure a sufficiently large area for the surface contact with the adjacent metal powder, so that the gap between those metal powders can be unfailingly reduced and the filling property can be satisfactorily enhanced.

The metal powder preferably has any one shape of an approximate cube, an approximate rectangular parallelepiped, an approximate triangular pyramid and approximate an quadrangular pyramid and the accessible surface of the metal powder preferably has any one shape of an approximate square, an approximate rectangle and an approximate triangle.

In this case, the metal powder when filled can be increased in the area for the surface contact with the adjacent metal powder, so that the gap between metal powders can be greatly reduced and the filling property can be more enhanced.

As for the shape of each accessible surface, in the case where the shape of the metal powder is an approximate cube, all surfaces have an approximately square shape; in the case of an approximate rectangular parallelepiped, the surfaces have an approximately square or approximately rectangular shape; in the case of an approximate triangular pyramid, all surfaces have an approximately triangular shape; and in the case of an approximate quadrangular pyramid, one surface has an approximately square or approximately rectangular shape and the remaining four surfaces have an approximately triangular shape.

Also, one side of the metal powder preferably has a length of 10 to 500 μm.

In this case, the metal powder when filled can efficiently make surface contact with the adjacent metal powder, so that the effect of reducing the gap between those metal powders and enhancing the filling property can be effectively exerted.

Assuming that the tap density of the metal powder is A and the density of the metal powder is B, the tapped filling rate represented by (A/B)×100(%) is preferably 60% or more.

In this case, the metal powder comes to have high filling property, so that the green compact obtained by using the metal powder can be satisfactorily and unfailingly increased in the density.

Incidentally, the tap density of the metal powder is measured by the following method. While charging the metal powder in an arbitrary weight a into a measuring cylinder, an operation of freely dropping the measuring cylinder from a height of 10 mm and an operation of pulling it up are repeated, thereby applying vibration to the bottom of the measuring cylinder. After the completion of charging of the metal powder, vibration is further applied 20 times to the bottom of the measuring cylinder. The volume b of the metal powder filled is measured and the a/b value is defined as the tap density A. Also, in the measurement of tap density, Tap Denser (KYT-2000, manufactured by Seishin Enterprise Co., Ltd.) or the like may be used.

The metal powder preferably has fine irregularities on the surface and the depth in the concave part of the irregularities is preferably 10% or less of the outermost diameter of the metal powder.

In this case, the mechanical connection of those metal powders to each other during compacting can be enhanced, so that the green compact obtained by using the metal powder can be increased in the strength.

If the depth in the concave part of the irregularities exceeds 10%, the gap between metal powders is increased when filled and the filling property may decrease.

The depth of the concave part is preferably from 1 to 50 μm.

In this case, the mechanical connection of the metal powders to each other during compacting can be more strengthened.

The metal powder is preferably any one metal of Fe type, Fe—Al type, Fe—Si type, Fe—Al—Si type, Fe—Co type and Fe—Ni type.

In this case, the green compact obtained by using the metal powder comes to have magnetic characteristics. Also, the green compact has a high density and therefore, the magnetic flux density as one of magnetic characteristics becomes high, so that when the green compact with excellent magnetic characteristics is applied as a powder magnetic core to a core, a solenoid or the like for motors, the performance can be enhanced. Incidentally, in this case, the performance can be more enhanced by using a metal powder having formed on the surface thereof an insulating film.

That is, the metal powder is preferably a powder for powder magnetic cores, with the surface being coated with an insulating film.

In this case, a high-density powder magnetic core can be obtained by compacting the powder for powder magnetic cores, which is the above-described metal powder. The thus-obtained high-density powder magnetic core comes to have a high magnetic flux density.

Furthermore, in the present invention, the compacting can be performed at a pressure lower than ever, so that the metal powder can be reduced in the deformation (distortion) generated due to pressure imposed at the compacting and the hysteresis loss of the powder magnetic core after compacting can be decreased. At the same time, the deformation/rupture of the insulting film coated on the surface of the metal powder can also be reduced and the insulation between metal powders can be satisfactorily ensured. By virtue of the presence of this insulating film, the powder magnetic core after compacting can be decreased in the eddy loss and a powder magnetic core with low iron loss can be obtained.

In this way, a powder magnetic core satisfying both high magnetic flux density and low iron loss can be obtained. In the case where the metal powder is a powder for powder magnetic cores, the preferred conditions are described below.

The metal powder preferably has an outermost diameter of 500 μm or less.

In this case, the effect of enhancing the filling property of the metal powder at the compacting can be satisfactorily and unfailingly obtained.

The thickness of the insulating film is preferably from 10 to 1,000 nm.

In this case, the insulation between metal powders at the compacting can be satisfactorily ensured.

The insulating film is preferably a ceramic film, a resin film or a mixed film of ceramic and resin.

In this case, the insulation between metal powders at the compacting can be satisfactorily ensured.

The ceramic film preferably comprises at least one or more member selected from the group consisting of alumina, silica, magnesia, zirconia, titania, boron nitride and silicon nitride.

In this case, the insulating film comes to have a high volume resistivity and therefore, the insulation between the metal powders can be enhanced.

The resin film preferably comprises at least one or more members selected from the group consisting of a silicone resin, a polyimide resin, a polyphenylene sulfide resin, a phenol resin, a polyether ketone-based resin, a silicone resin and a silane coupling agent.

In this case, the insulating film comes to have high electrical insulation and therefore, the insulation between the metal powders can be enhanced.

In the second invention, the metal powder is preferably filled by vibrating the die in the filling step.

In this case, the metal powder can be filled in a sufficiently dense state, whereby the density of the obtained green compact can be increased.

Also, in the filling step, the die is preferably vibrated by using an ultrasonic generator.

In this case, the metal powder can be filled in a denser state, whereby the density of the obtained green compact can be more increased.

As for the metal powder, a powder for powder magnetic cores, with the surface being coated with an insulating film, may also be used.

In this case, a high-density powder magnetic core can be obtained by compacting the powder for powder magnetic cores, which is the above-described magnetic metal. The thus-obtained high-density powder magnetic core comes to have a high magnetic flux density.

Furthermore, in the production method of the present invention, the compacting can be performed at a pressure lower than ever, so that deformation (distortion) of the metal powder and deformation/rupture of the insulating film can be reduced. At the same time, the hysteresis loss and eddy loss of the powder magnetic core after compacting can be decreased and a powder magnetic core with low iron loss can be thereby obtained.

In this way, a powder magnetic core satisfying both high magnetic flux density and low iron loss can be obtained.

In the metal powder which is a powder for powder magnetic cores, with the surface being coated with an insulating film, assuming that the surface area before the compacting step is S1 and the surface area after the compacting step is S2, the value of (S1−S2)/S1 is preferably 0.2 or less.

In this case, deformation (distortion) of the metal powder and deformation/rupture of the insulating film can be reduced, whereby the iron loss of the obtained powder magnetic core can be more decreased.

In the third invention, the green compact preferably has a relative density of 95% or more.

In this case, the green compact comes to a sufficiently high density with less voids.

The green compact may be a powder magnetic core obtained by compacting a powder for powder magnetic cores, which is the above-described metal powder with the surface being coated with an insulating film.

In this case, the green compact can satisfy both high magnetic flux density and low iron loss.

In the case where the green compact is a powder magnetic core, the preferred conditions are described below.

The green compact preferably has a density of 7.4 Mg/m³ or more.

In this case, the green compact comes to have a sufficiently high density, whereby the magnetic flux density of the green compact becomes sufficiently high. The green compact preferably has a saturation magnetic flux density of 1.6 T or more.

In this case, the green compact comes to have a sufficiently high magnetic flux density.

The green compact can be used, as a high-density sintered part, for gear parts (e.g., sprocket), pump parts (e.g., rotor, vane) and the like.

Also, in the case where the green compact is a powder magnetic core, the green compact can be used for a motor core, a solenoid core, a reactor and the like.

EXAMPLES

The present invention will be further described with reference to the examples thereof.

Example 1

The metal powder according to the example of the present invention is described below.

The metal powder 1 of this example is, as shown in FIG. 1, a metal powder which is a raw material for obtaining a green compact by compacting. The metal powder 1 has a plurality of accessible surfaces 11 allowing for surface contact of adjacent metal powders 1 with each other when filled.

This is described in detail below.

The metal powder 1 of this example is pure iron (Fe) and, as shown in FIG. 1, has an approximately cubic shape with a one-side length of 100 μm. The outermost diameter of the metal powder 1 is 170 μm. Also, the metal powder 1 has six accessible surfaces 11 allowing for surface contact of adjacent metal powders 1 with each other when filled. The accessible surface 11 has an approximately square shape. Although a cubic metal powder 1 which is most ideal is shown in FIG. 1 for the sake of simplifying the drawing, the metal powder is practically not such an exact cube but has a shape with the corners rounded in many cases.

As shown in FIG. 2, fine irregularities are provided on the surface 10 of the metal powder 1. In the figure, an irregularity shape having square corners, which is most ideal, is shown for the sake of simplifying the drawing, but as shown in FIG. 3, the irregularity practically has a corner-rounded shape in many cases.

Also, when the surface 10 of the metal powder 1 was examined by using a three-dimensional shape measuring microscope (VK-8500, manufactured by Keyence Corp.), the depth D in the concave part 101 of those irregularities was about 5 μm.

In the metal powder 1 of this example, the total area of six accessible surfaces 11 is 90% of the entire surface area (the entire area of the surface 10) of the metal powder 1, that is, the metal powder 1 when filled can make surface contact with the adjacent metal powder 1 at many parts of the surface 10 (see, FIG. 4).

Assuming that the tap density of the metal powder 1 is A and the density of the metal powder 1 is B, the tapped filling rate represented by (A/B)×100(%) was 90% (A=7.06 Mg/m³, B=7.85 Mg/m³).

The tap density of the metal powder 1 of this example was measured by using Tap Denser (KYT-2000, manufactured by Seishin Enterprise Co., Ltd.). To speak specifically, while charging the metal powder 1 in a weight of 300 g (=a) into a 100 ml-volume measuring cylinder, an operation of freely dropping the measuring cylinder from a height of 10 mm and an operation of pulling it up are repeated, thereby applying vibration to the bottom of the measuring cylinder. After the completion of charging of the metal powder 1, vibration is further applied 20 times to the bottom of the measuring cylinder, and the volume b of the metal powder 1 filled is measured. In this example, the volume was 42.5 ml (=b). Then, the tap density A (=a/b) was determined.

The production method of the metal powder 1 is briefly described below. As for the production method of the metal powder 1 of this example, a cast-molding method was used.

First, pure iron working out to a material constituting the metal powder 1 is melted to prepare a molten iron. This molten iron is then cast into a casting mold having a concave part in an approximately cubic shape with a one-side length of 100 μm. Incidentally, fine irregularities are provided on the inner surface of the casting mold and the depth in the concave part of the inner surface is 5 μm. Subsequently, the molten iron is cooled and solidified in the casting mold. Finally, a metal powder 1 is taken out from the casting mold.

In this way, a metal powder 1 having an approximately cubic shape with a one-side length of 100 μm is obtained.

In the production method of the metal powder 1 of this example, the construction material of the casting mold, which can be used, is a ceramic such as silicon nitride, alumina and magnesia, or a metal such as iron. From the standpoint of ease in taking out the metal powder 1 from the casting mold, a ceramic having a small thermal expansion coefficient is preferably used. Also, in view of thermal shock when casting the molten iron into the casting mold, silicon nitride, as a ceramic, is preferred.

As for the production method of the metal powder 1, in addition to the cast-molding method of this example, an extrusion-molding method, a draw-molding method and the like may also be used. For example, an angular wire rod having a 100 μm-square cross-section is molded by extrusion or draw-molding and cut at a pitch of 100 μm, whereby the same metal powder as the metal powder 1 of this example can be obtained.

The production method of a green compact 2 (see FIG. 5) using the above-described metal powder 1 is described below.

The production method of a green compact 2 of this example comprises a charging step of charging the metal powder 1 into a predetermined die, a filling step of filling the metal powder 1 in the die, and a compacting step of compacting the metal powder 1 to obtain a green compact 2.

This is described in detail below.

In the charging step, the metal powder 1 is charged into a die having a shape of the green compact 2 to be molded.

Subsequently, in the filling step, the die is vibrated by using an ultrasonic generator to densely fill the metal powder 1. At this time, as shown in FIG. 4, the metal powders 1 are aligned as neatly as possible to bring the accessible surfaces 11 into surface contact with each other. The filling is performed to give as small a gap as possible between the metal powders 1. The filling density of the metal powder 1 in the die after filling was 4.6 Mg/m³. Incidentally, in FIG. 4, the metal powder 1 filled in a die is shown by extracting a part thereof. In the figure, the ideal filled state is shown, but in practice, there may be produced a portion where the accessible surfaces 11 are not contacted with each other.

Subsequently, in the compacting step, a pressure of 800 MPa is applied to the metal powder 1 after filling so as to effect compacting.

In this way, a green compact 2 of FIG. 5 is obtained.

The green compact 2 produced by the above-described production method is described below.

As shown in FIG. 5, the green compact 2 of this example has a cylindrical shape (ring shape). The density of the green compact 2 was 7.8 Mg/m ³, and the relative density was 99.4%.

The green compact 2 can be molded into various shapes according to usage by changing the shape of the die.

The operational effect of the metal powder 1 of this example is described below.

The metal powder 1 of this example has a plurality of accessible surfaces 11. By virtue of this constitution, the metal powder 1 when filled can unfailingly make surface contact with the adjacent metal powder 1 at a plurality of accessible surfaces 11, whereby the gap formed between metal powders 1 filled can be reduced and the filling property of the metal powder 1 can be enhanced.

The green compact 2 obtained by densely filling this metal powder 1 having high filling property and compacting it in the state of the gap between metal powders 1 being reduced comes to have a high density with the presence of less voids after compacting.

Also, as the metal powder 1 has a high filling property, a high pressure for filling the gap between metal powders 1 need not be applied at compacting, so that a high-density green compact 2 can be obtained by performing the compacting at a pressure lower than ever.

In this example, the total area of the accessible surfaces 11 is 90% of the entire surface area of the metal powder 1. Therefore, the metal powder 1 when filled can ensure a sufficiently large area for the surface contact with the adjacent metal powder 1, whereby the gap between the metal powders 1 can be unfailingly reduced and the filling property can be satisfactorily enhanced.

The metal powder 1 has an approximately cubic shape with a one-side length of 100 μm and has approximately square accessible surfaces 11. Therefore, the metal powder 1 when filled can efficiently make surface contact with the adjacent metal powder 1 and at the same time, can be increased in the area for the surface contact, whereby the gap between metal powders 1 can be greatly reduced and the filling property can be more enhanced.

The tapped filling rate of the metal powder 1 is 90% as measured by the method described above and therefore, the metal powder 1 comes to have high filling property, whereby the green compact 2 obtained by using the metal powder 1 can be satisfactorily and unfailingly increased in the density.

The metal powder 1 has fine irregularities on the surface 10 and the depth D in the concave part 101 of the irregularities is 10% or less of the outermost diameter of the metal powder 1 and is 5 μm. Therefore, the mechanical connection of those metal powders 1 to each other during compacting can be enhanced, whereby the green compact 2 obtained by using the metal powder 1 can be increased in the strength.

In the production method of this example, in the filling step, the metal powder 1 is filled by vibrating the die with use of an ultrasonic generator and therefore, the metal powder 1 can be filled in a denser state, whereby the density of the obtained green compact 2 can be more increased.

In this way, according to this example, a metal powder having high filling property and capable of realizing a high-density green compact, a green compact using the same, and a production method thereof can be provided.

In this example, pure iron (Fe) is used as the metal powder 1, but other than this, a metal of Fe—Al type, Fe—Si type, Fe—Al—Si type, Fe—Co type, Fe—Ni type or other various species may be used.

Also, an approximate cube is employed as the shape of the metal powder 1, but other various shapes may be employed. For example, a shape such as approximate rectangular parallelepiped (FIG. 6), approximate triangular pyramid (FIG. 7) or approximate quadrangular pyramid (FIG. 8) may be employed.

In the case of an approximate rectangular parallelepiped, the accessible surface 11 has an approximately square or approximately rectangular shape; in the case of an approximate triangular pyramid, all surfaces have an approximately triangular shape; and in the case of an approximate quadrangular pyramid, one surface has an approximately square or approximately rectangular shape and the remaining four surfaces have an approximately triangular shape.

Even when these shapes are employed, the total area of the accessible surfaces 11 is preferably 70% or more of the entire surface area of the metal powder 1. The tapped filling rate is preferably 60% or more, more preferably 70% or more.

Example 2

This example demonstrates an application example of the green compact 2 obtained by using the metal powder 1 of Example 1.

The green compact 2 can be used, as a high-density sintered part, for a sprocket (FIG. 9) of gear parts, a rotor (FIG. 10) of pump parts, a vane and the like.

Also, the green compact 2 can be molded into various shapes according to usage and can be applied to various materials.

Example 3

This example demonstrates a case where a powder for powder magnetic cores, with the surface being coated with an insulating film, is used as the metal powder.

The powder 3 for powder magnetic cores of this example is a metal powder which becomes a raw material for obtaining, as shown in FIG. 11, a powder magnetic core by compacting. The powder 3 for powder magnetic cores has a plurality of accessible surfaces 11 allowing for surface contact with the adjacent powder 3 for powder magnetic cores, and the surface 10 thereof is coated with an insulating film 12.

This is described in detail below.

The powder 3 for powder magnetic cores of this example is pure iron (Fe) and, as shown in FIG. 11, has an approximately cubic shape with a one-side length of 100 μm. The outermost diameter of the powder 3 for powder magnetic cores is 170 μm. Also, the powder 3 for powder magnetic cores has six accessible surfaces 11 allowing adjacent powders 3 for powder magnetic cores to make surface contact with each other when filled. The accessible surface 11 has an approximately square shape. Although a cubic powder 3 for powder magnetic cores, which is most ideal, is shown in FIG. 11 for the sake of simplifying the drawing, the powder for powder magnetic cores is practically not such an exact cube but often has a shape with the corners rounded.

As shown in FIG. 12, fine irregularities are provided on the surface 10 of the powder 3 for powder magnetic cores. In the figure, an irregularity shape having square corners, which is most ideal, is shown for the sake of simplifying the drawing, but as shown in FIG. 13, the irregularity practically has a corner-rounded shape in many cases.

Also, when the surface 10 of the powder 3 for powder magnetic cores was examined by the same method as in Example 1, the depth d in the concave part 101 of those irregularities was about 5 μm.

As shown in FIGS. 11 to 13, the surface 10 of the powder 3 for powder magnetic cores is entirely coated with an insulating film 12 comprising a silicone resin and having an average thickness of 50 nm.

In the case of the powder 3 for powder magnetic cores of this Example, the total area of six accessible surfaces 11 is 90% of the entire surface area (the entire area of the surface 10) of the powder 3 for powder magnetic cores, that is, the powder 3 for powder magnetic cores when filled can make surface contact with the adjacent powder 3 for powder magnetic cores at many parts of the surface 10 (see, FIG. 14).

Assuming that the tap density of the powder 3 for powder magnetic cores is A and the density of the powder 3 for powder magnetic cores is B, the tapped filling rate represented by (A/B)×100(%) was 90% (A=7.02 Mg/m³, B=7.80 Mg/m³). Incidentally, the tap density A of the powder 3 for powder magnetic cores was measured by the same method as in Example 1.

The production method of the powder 3 for powder magnetic cores is briefly described below. As for the production method of this example, a cast-molding method was used similarly to Example 1.

First, pure iron working out to a material constituting the powder 3 for powder magnetic cores is melted to prepare a molten iron. This molten iron is then cast into a casting mold having a concave part in an approximately cubic shape with a one-side length of 100 μm. Incidentally, fine irregularities are provided on the inner surface of the casting mold and the depth in the concave part of the inner surface is 10 μm. Subsequently, the molten iron is cooled and solidified in the casting mold. Finally, a powder 3 for powder magnetic cores is taken out from the casting mold, whereby a powder 3 for powder magnetic cores having an approximately cubic shape with a one-side length of 100 μm is obtained.

Throughout the surface 10 of the obtained powder 3 for powder magnetic cores, an insulating film 12 comprising a silicone resin and having an average thickness of 50 nm is formed, for example, by a method of spray-coating a silicone resin solution or a method of dipping the powder 3 for powder magnetic cores in a silicone resin solution and pulling it up.

In this way, a powder 3 for powder magnetic cores of FIG. 11 is obtained.

In the production method of this example, the construction material of the casting mold, which can be used, is a ceramic such as silicon nitride, alumina and magnesia, or a metal such as iron. From the standpoint of ease in taking out the powder 3 for powder magnetic cores from the casting mold, a ceramic having a small thermal expansion coefficient is preferably used. Also, in view of thermal shock when casting the molten iron into the casting mold, silicon nitride, as a ceramic, is preferred.

As for the production method of the powder 3 for powder magnetic cores, in addition to the cast-molding method of this Example, an extrusion-molding method, a draw-molding method and the like may also be used. For example, an angular wire rod having a 100 μm-square cross-section is molded by extrusion or draw molding and cut at a pitch of 100 μm, whereby the same powder as the powder 3 for powder magnetic cores of this example can be obtained.

As for the method for coating an insulating film 12, in the case where the insulating film 12 is a resin film, for example, a method of mixing the powder 3 for powder magnetic cores with a resin powder and attaching the resin powder to the surface 10 of the powder 3 for power magnetic cores, or a method of spraying a resin in a solution state on the powder 3 for powder magnetic cores can be used.

In the case where the insulating film 12 is a ceramic film, for example, a method of coating the surface 10 of the powder 3 for powder magnetic cores with the film through a chemical reaction of alkoxide, sol-gel or the like, a method of spraying a ceramic fine powder on the powder 3 for powder magnetic cores, a PVD method or a CVD method can be used.

In the case where the insulating film 12 is a mixed film of ceramic and resin, for example, a method of dispersing a ceramic powder in a resin in a solution state and spraying the resulting mixed solution on the powder 3 for powder magnetic cores can be used.

The production method of a powder magnetic core 4 (see, FIG. 15) using the above-described powder 3 for powder magnetic cores is described below.

The production method of a powder magnetic core 4 of this example comprises a charging step of charging the powder 3 for powder magnetic cores into a predetermined die, a filling step of filling the powder 3 for powder magnetic cores in the die, and a compacting step of compacting the powder 3 for powder magnetic cores to obtain a powder magnetic core 4.

This is described in detail below.

In the charging step, the powder 3 for powder magnetic cores is charged into a die having a shape of the powder magnetic core 4 to be molded.

Subsequently, in the filling step, the die is vibrated by using an ultrasonic generator to densely fill the powder 3 for powder magnetic cores. At this time, as shown in FIG. 14, the powders 3 for powder magnetic cores are aligned to bring the accessible surfaces 11 into surface contact with each other. The filling is performed to give as small a gap as possible between the powders 3 for powder magnetic cores. The filling density of the powder 3 for powder magnetic cores in the die after filling was 5.5 Mg/m³. Incidentally, in FIG. 14, the powder 3 for powder magnetic cores filled in a die is shown by extracting a part thereof. In the figure, the ideal filled state is shown, but in practice, there may be produced a portion where the accessible surfaces 11 are not contacted with each other.

Subsequently, in the compacting step, a pressure of 800 MPa is applied to the powder 3 for powder magnetic cores after filling so as to effect compacting.

In this way, a powder magnetic core 4 of FIG. 15 is obtained.

In order to evaluate the deformation (distortion) of the powder 3 for powder magnetic cores, which is generated due to a pressure imposed during compacting in the production method of this example, the surface area of the powder 3 for powder magnetic cores was measured before and after compacting. The surface area S1 before compacting was 755 m²/m³, the surface area S2 after compacting was 650 m²/m³, and the rate of change in the surface area between before and after compacting, represented by (S1−S2)/S1, was 0.14.

The powder magnetic core 4 produced by the above-described production method is described below.

As shown in FIG. 15, the powder magnetic core 4 of this example has a cylindrical shape (ring shape) and is used as a stator core for motors. The density of the powder magnetic core 4 was 7.70 Mg/m³ and the relative density was 98.7%. Also, the saturation magnetic flux density measured by a BH analyzer was 1.82 T and the iron loss in an AC magnetic field at 400 Hz was 40 W/kg.

The powder magnetic core 4 can be molded into various shapes, according to usage, by changing the shape of the die.

The operational effect of the powder 3 for powder magnetic cores of this example is described below.

The powder 3 for powder magnetic cores of this example has a plurality of accessible surfaces 11 allowing for surface contact with the adjacent powder 3 for powder magnetic cores when filled. That is, the powder 3 for powder magnetic cores when filled can unfailingly make surface contact with the adjacent powder 3 for powder magnetic cores at a plurality of accessible surfaces 11, whereby the gap formed between powders 3 for powder magnetic cores filled can be reduced and the filling property of the powder 3 for powder magnetic cores can be enhanced.

The powder magnetic core 4 obtained by densely filling this powder 3 for powder magnetic cores having high filling property and compacting it in the state of the gap between powders 3 for powder magnetic cores being reduced comes to have a high density with the presence of less voids after compacting, and the powder magnetic core 4 having such a high density comes to have a high magnetic flux density.

Also, the powder 3 for powder magnetic cores has high filling property and therefore, a high pressure for filling the gap between powders 3 for powder magnetic cores need not be applied at compacting. In other words, a powder magnetic core 4 with high density and high magnetic flux density can be obtained by performing the compacting at a pressure lower than ever.

Furthermore, since the compacting can be performed at a pressure lower than ever, the powder 3 for powder magnetic cores can be reduced in the deformation (distortion) generated due to a pressure imposed during compacting, and the hysteresis loss of the powder magnetic core 4 after compacting can be decreased. At the same time, the deformation/rupture of the insulating film 12 coated on the surface 10 of the powder 3 for powder magnetic cores can also be decreased, and the insulation between powders 3 for powder magnetic cores can be satisfactorily ensured. By virtue of the presence of this insulating film 12, the powder magnetic core 4 after compacting can be decreased in the eddy loss and a powder magnetic core 4 with low iron loss can be obtained.

In this example, the total area of accessible surfaces 11 is 90% of the entire surface area of the powder 3 for powder magnetic cores. Therefore, the powder 3 for powder magnetic cores when filled can ensure a sufficiently large area for the surface contact with the adjacent powder 3 for powder magnetic cores, whereby the gap between the powders 3 for powder magnetic cores can be unfailingly reduced and the filling property can be satisfactorily enhanced.

The powder 3 for powder magnetic cores has an approximately cubic shape with a one-side length of 100 μm and has approximately square accessible surfaces 11. Therefore, the powder 3 for powder magnetic cores when filled can efficiently make surface contact with the adjacent powder 3 for powder magnetic cores and at the same time, can be increased in the area for the surface contact, whereby the gap between powders 3 for powder magnetic cores can be greatly reduced and the filling property can be more enhanced.

The tapped filling rate of the powder 3 for powder magnetic cores is 90% as measured by the method described above and therefore, the powder 3 for powder magnetic cores comes to have high filling property, whereby the powder magnetic core 4 obtained by using the powder 3 for powder magnetic cores can be satisfactorily and unfailingly increased in the magnetic flux density and reduced in the iron loss.

The powder 3 for powder magnetic cores has fine irregularities on the surface 10 and the depth d in the concave part 101 of the irregularities is 10% or less of the outermost diameter of the powder 3 for powder magnetic cores and is 5 μm. Therefore, the mechanical connection of those powders 3 for powder magnetic cores to each other during compacting can be enhanced, whereby the powder magnetic core 4 obtained by using the powder 3 for powder magnetic cores can be increased in the strength.

The insulating film 12 is a resin film comprising a silicone resin and having a thickness of 50 nm and therefore, the insulating film 12 comes to have high electrical insulation, whereby the insulation between powders 3 for powder magnetic cores can be enhanced.

In the production method of the powder magnetic core 4 of this example, in the filling step, the powder 3 for powder magnetic cores is filled by vibrating the die with use of an ultrasonic generator and therefore, the powder 3 for powder magnetic cores can be filled in a denser state, whereby the obtained powder magnetic core 4 can be more increased in density and in magnetic flux density.

In the powder 3 for powder magnetic cores, assuming that the surface area before the compacting step is S1 and the surface area after the compacting step is S2, the value of (S1−S2)/S1 is 0.14. Therefore, deformation (distortion) of the powder 3 for powder magnetic cores and deformation/rupture of the insulating film 12 can be satisfactorily reduced, whereby the powder magnetic core 4 can be more decreased in the iron loss.

Also, the powder magnetic core 4 produced by the production method of the powder magnetic core 4 of this Example has a relative density of 98.7%. Therefore, the powder magnetic core 4 comes to have a high density with less voids.

Furthermore, the powder magnetic core 4 has a density of 7.70 Mg/m³ and a saturation magnetic flux density of 1.82 T. Therefore, the powder magnetic core 4 comes to have a satisfactorily high density and at the same time, a high magnetic flux density.

In this way, according to this example, a powder 3 for powder magnetic cores, capable of giving a powder magnetic core having high filling property and satisfying both high magnetic flux density and low iron loss, can be provided. The powder magnetic core 4 obtained by using this powder 3 for powder magnetic cores comes to satisfy both high magnetic flux density and low iron loss.

In this example, pure iron (Fe) of this example is used as the material constituting the powder 3 for powder magnetic cores, but other than this, a metal of Fe—Al type, Fe—Si type, Fe—Al—Si type, Fe—Co type, Fe—Ni type or the like may be used.

Furthermore, as regards the insulating film 12, for example, a ceramic film or a mixed film of ceramic and resin may be used other than the resin film of this Example comprising a silicone resin.

As for the resin film, a film comprising one resin species or a plurality of resin species, such as the silicone resin of this Example, a polyimide resin, a polyphenylene sulfide resin, a phenol resin, a polyether ketone-based resin, a silicone resin and a silane coupling agent, may be used.

As for the ceramic film, a film comprising one ceramic species or a plurality of ceramic species such as alumina, silica, magnesia, zirconia, titania, boron nitride and silicon nitride may be used.

Also, an approximate cube is employed as the shape of the powder 3 for powder magnetic cores of this Example, but other various shapes may be used. For example, a shape such as approximate rectangular parallelepiped (see, FIG. 6), approximate triangular pyramid (see, FIG. 7) or approximate quadrangular pyramid (see, FIG. 8) may be employed.

In the case of an approximate rectangular parallelepiped, the accessible surfaces 11 have an approximately square or approximately rectangular shape; in the case of an approximate triangular pyramid, all surfaces have an approximately triangular shape; and in the case of an approximate quadrangular pyramid, one surface has an approximately square or approximately rectangular shape and the remaining four surfaces have an approximately triangular shape.

Even when these shapes are employed, the total area of the accessible surfaces 11 is preferably 70% or more of the entire surface area of the powder 3 for powder magnetic cores. The tapped filling rate is preferably 60% or more, more preferably 70% or more.

The powder magnetic core 4 of this example is molded into the shape of a stator core for motors but may be molded into various shapes according to usage by changing the shape of the die for compacting the powder 3 for powder magnetic cores. Other than the usage described above, the powder magnetic core 4 can be applied to a rotor core for motors, a noise filter, a reactor and the like. 

1. A metal powder as a raw material for obtaining a green compact by compacting, said metal powder having a plurality of accessible surfaces allowing for surface contact of adjacent metal powders with each other when filled.
 2. The metal powder as claimed in claim 1, wherein the total area of said accessible surfaces is 70% or more of the entire surface area of said metal powder.
 3. The metal powder as claimed in claim 1, wherein said metal powder has any one shape of an approximate cube, an approximate rectangular parallelepiped, an approximate triangular pyramid and an approximate quadrangular pyramid and said accessible surface of said metal powder has any one shape of an approximate square, an approximate rectangle and an approximate triangle.
 4. The metal powder as claimed in claim 3, wherein one side of said metal powder has a length of 10 to 500 μm.
 5. The metal powder as claimed in claim 1, wherein assuming that the tap density of said metal powder is A and the density of said metal powder is B, the tapped filling rate represented by (A/B)×100(%) is 60% or more.
 6. The metal powder as claimed in claim 1, wherein said metal powder has fine irregularities on the surface and the depth in the concave part of said irregularities is 10% or less of the outermost diameter of said metal powder.
 7. The metal powder as claimed in claim 6, wherein the depth of said concave part is from 1 to 50 μm.
 8. The metal powder as claimed in claim 1, wherein said metal powder is any one metal of Fe type, Fe—Al type, Fe—Si type, Fe—Al—Si type, Fe—Co type and Fe—Ni type.
 9. The metal powder as claimed in claim 1, wherein said metal powder is a powder for powder magnetic cores, with the surface being coated with an insulating film.
 10. The metal powder as claimed in claim 9, wherein said metal powder has an outermost diameter of 500 μm or less.
 11. The metal powder as claimed in claim 9, wherein the thickness of said insulating film is from 10 to 1,000 nm.
 12. The metal powder as claimed in claim 9, wherein said insulating film is a ceramic film, a resin film or a mixed film of ceramic and resin.
 13. The metal powder as claimed in claim 12, wherein said ceramic film comprises at least one or more members selected from the group consisting of alumina, silica, magnesia, zirconia, titania, boron nitride and silicon nitride.
 14. The metal powder as claimed in claim 12, wherein said resin film comprises at least one or more member(s) selected from the group consisting of a silicone resin, a polyimide resin, a polyphenylene sulfide resin, a phenol resin, a polyether ketone-based resin, a silicone resin and a silane coupling agent.
 15. A method for producing a green compact by compacting the metal powder claimed in claim 1, said method comprising: a charging step of charging said metal powder into a predetermined die, a filling step of filling said metal powder in said die, and a compacting step of compacting said metal powder to obtain said green compact.
 16. The method for producing a green compact as claimed in claim 15, wherein in said filling step, said metal powder is filled by vibrating said die.
 17. The method for producing a green compact as claimed in claim 16, wherein in said filling step, said die is vibrated by using an ultrasonic generator.
 18. The method for producing a green compact as claimed in claim 15, wherein said metal powder is a powder for powder magnetic cores, with the surface being coated with an insulating film, and assuming that the surface area of said metal powder before said compacting step is S1 and the surface area after said compacting step is S2, the value of (S1−S2)/S1 is 0.2 or less.
 19. A green compact produced by the method claimed in claim
 15. 20. The green compact as claimed in claim 19, wherein said green compact has a relative density of 95% or more.
 21. The green compact as claimed in claim 19, wherein said green compact is a powder magnetic core obtained by compacting a powder for powder magnetic cores, which is said metal powder with the surface being coated with an insulating film and has a density of 7.4 Mg/m³ or more.
 22. The green compact as claimed in claim 21, wherein said green compact has a saturation magnetic flux density of 1.6 T or more. 